1. Field of the Invention
The conventional composite fiberboards, typical of those used for acoustical ceilings, are composed of mineral wool, perlite and newsprint as the main ingredients, newsprint being primarily cellulosic fibers. These materials are formed into boards from aqueous slurries using an organic binder such as starch.
The mineral wool may be composed of fibers of rock wool or basalt wool. It is also possible to use glass fibers, alone, or mixed with the mineral wool. The fibers, in general, have a diameter of 3 to 6 microns. The fibers may be used in the "sized" or "naked" state. Sizing agents such as mineral oils or acrylic polymer dispersions may be employed. These fibers contribute to the structural integrity and strength of the board.
The perlite is a volcanic glass ore composed of aluminum, calcium or other alkaline earth silicate. Prior to use in the fiberboard process, the perlite ore is expanded at high temperatures to obtain densities of 2 to 10 pounds/cubic foot (pcf), preferably 3 to 7 pcf. Perlite contributes to the bulk and hardness of the board. The perlite may also be coated with hydrophobic coatings such as silicones, sizing agents and the like.
The third important ingredient is the so-called "newsprint". Specifically, the newsprint is composed of cellulosic fibers. These fibers contribute to the wet strength of the board as it is converted from the slurry to the substantially solid cake enroute to becoming the board. Hereinafter, these fibers will be referred to as "cellulosic newsprint" fibers.
The mixture may also contain fillers, flame-proofing agents, pigments, water repellants, etc. The fillers employed are usually finely divided clays, e.g., kaolin, ball clay, etc.
In the process of preparing the board, the ingredients are mixed together with the amount of water necessary to provide slurry consistency in conventional mixing and holding equipment. Additional water and "dry broke" may be added to the slurry prior to the addition of the starch binder. The starch is employed in amounts as high as about 15%, based on the three primary ingredients. The "dry broke" is predominately recycled board material that may have been rejected or cut from the commercially acceptable boards, as well as other waste products.
The slurry containing less than 5% solids is pumped to a head box and, immediately thereafter, dewatered by being transferred onto the board forming wire of a Fourdrinier. Suction is usually applied, sometimes along with pressure, to assist in drainage and compaction using conventional means.
One disadvantage of these conventional sound-insulating boards made using starch a the binder is their moisture sensitivity. Their tendency to sag in a moist atmosphere may make it necessary to coat or impregnate the back and/or face of the boards with, for example, thermosetting plastics or other moisture-resistant compositions. Not only does this add the extra coating step, but further drying and heating becomes necessary. In short, a more expensive board results.
Overcoming the sag resistance problem by substituting latex compositions for the starch binders as set forth in U.S. Pat. Nos. 4,863,979 and 4,963,603, and using the above-described conventional process involving the Fourdrinier and the conventional head-box does not provide particularly high acoustical properties in the ultimate composite board.
Producing a highly acoustical composite board (a board that will contribute to a substantial reduction in the noise level), currently relies upon the use of man-made fibers (fiberglass, mineral wool, etc.) as the predominant component in the board. The resulting board is highly porous consisting of a large number of small pores. The man-made fibers tend to be more resistant to compression during the free drainage and vacuum sections of the Fourdrinier and the cylinder machines that are sometimes used. Therefore, the fine pore structure remains intact to provide the board with its acoustical properties. However, this method, whether or not it utilizes a significant amount of man-made fibers, results in a wet mat that has a significant density gradient face to back and, subsequently, poor felting on the face and tight felting on the back.
Increased board thickness also tends to improve acoustical properties; the thickness being increased by increasing the mass of the board. However, thicker boards require more stock per square foot and, obviously, are more expensive.
One of the objects of this invention is to produce highly acoustical boards that are less expensive and which use relatively low percentages of man-made fibers and relatively high percentages of cellulose (newsprint) and perlite.
2. Description of the Prior Art
In U.S. Pat. No. 4,587,278, sound-insulating boards which are based on mineral fibers and thermoplastic binders are disclosed to overcome the moisture-sensitive disadvantages of the starch-bound board. The binders disclosed in this patent are polymers having glass transition temperatures from 38.degree. to 80.degree. C. These binders may be inadequate for the boards to retain dimensional stability without any substantial sag when exposed for prolonged periods at high temperatures with high humidity.
In U.S. Pat. No. 4,863,979, the applicant discloses the use of thermoplastic binders (latex compositions) having glass transition temperatures anywhere from 35.degree. C. to 115.degree. C. The boards disclosed contain newsprint, perlite, mineral wool and clay as well as the latex binder. They are manufactured by incorporating the latex binder into a previously prepared aqueous slurry of the other ingredients. The resulting boards, according to the disclosure, display acceptable strength and an improvement in "dimensional stability" as measured by the composite board's sag resistance.
U.S. Pat. No. 4,963,603 discloses a process for minimizing the amount of thermoplastic polymeric latex binder required to provide a commercially attractive composite board containing cellulosic newsprint along with mineral wool and perlite. The board displays acceptable strength and an unexpectedly high dimensional stability. The crux of the invented process is in selectively depositing the latex binder onto the cellulosic fibers prior to binding all of the board's components to form the ultimate board.
An object of this invention is to provide a composite board of mineral wool, perlite and 4-35% by weight of cellulosic newsprint fibers that displays low sag (less than 0.2 inches or 200 mils) when a 9.times.24 inch board, 0.6 inch thick, (the base board being sanded on the face and/or back) is supported at each end to form a 23 inch span and exposed to a temperature of 85.degree. F. and a relative humidity of 95% for 24 hours, a relatively low density gradient from face to back, i.e., no greater than 0.05 psf from face to back, and improved acoustical properties, i.e., a noise reduction coefficient (NRC) as measured by ASTM test designation C423-84a on the face and/or back of the base board, of at least about 0.60, preferably at least about 0.725.
Another object is to provide a process for manufacturing a composite board displaying minimal sag and excellent acoustical properties.